This exercise is particularly good for running type athletes because it builds good core stability by working the transverse abdominis (a deep abdominal muscle that needs to hold the right amount of tension to prevent excessive movement of the pelvis and lumbar spine during running).
This exercise helps also to maintain the appropriate tension while the legs move freely and alternately.
1. To begin, lie face up on the floor with both knees sharply bent and both your feet flat on the floor. Press your lower back into the floor.
2. As you concentrate on keeping your lower back pressed to the floor, lift your right leg until your right foot comes even with your left knee.
3. Next, lower the foot back to the floor. Repeat with the left leg.
Continue until you begin to feel an uncomfortable burn in your abs; usually about 20 reps per leg.